U.S. Pat. No. 6,223,594 B1 (JP-A-11-118559) discloses a thermal flow detecting apparatus for detecting airflow in an airflow passage. The flow detecting apparatus is a thermal flow meter for detecting airflow using a thermal resistive element as a hot wire.
As shown in FIG. 5, a conventional flow meter has an inner wall defining a bypass passage portion 917 being rectangular in cross section. The inner wall of the flow meter is provided partially with throttle portions 920 on both sides of the bypass passage portion 917. Each of the throttle portions 920 extends from an inlet end 925 toward an outlet end 926 with respect to the flow direction of the air. The outlet end 926 defines a throttle outlet cavity 921. Support members 927, 929 are provided in the throttle outlet cavity 921. Each of the support members 927, 929 is behind the corresponding throttle portion 920 defining a throttle surface in a convex surface when being viewed from an inlet of the throttle portions 920. A thermal resistive element 931 and a temperature sensing element 932 are provided in the bypass passage portion 917. The thermal resistive element 931 and the temperature sensing element 932 are supported using the support members 927, 929.
In this structure, the throttle portions 920 are capable of deflecting air, which passes through the bypass passage portion 917, from the support members 927, 929. The airflow can be smoothly rectified, so that the flow meter is capable of steadily detecting the airflow thereby outputting a stable detection signal. The flow meter has a thermal sensing element including the thermal resistive element 931 and the temperature sensing element 932 to measure the rectified airflow throughout a wide range between a small flow rate and a large flow rate, thereby transmitting the output signal indicating the airflow. In this structure, fluctuation in the output signal of the thermal sensing element can be reduced by rectifying the airflow.
The bypass passage portion 917 of the flow meter is inserted into the airflow passage to detect the airflow through the airflow passage. When the diameter of the bypass passage portion 917 is large, the bypass passage portion 917 causes large pressure drop in the airflow passage. Therefore, the diameter of the bypass passage portion 917 is preferably reduced to decrease pressure drop in the airflow passage. However, when the bypass passage portion 917 is reduced in inner diameter, a space for accommodating the thermal sensing element including the thermal resistive element 931 and the temperature sensing element 932 becomes small. The bypass passage portion 917 has therein a center portion where speed of airflow is substantially stable. In a structure in which the bypass passage portion 917 is reduced in inner diameter, the thermal sensing elements are hard to be arranged in the center of the bypass passage portion 917. Consequently, detection property of the flow meter may change when dispersion occurs in the position of the thermal sensing elements.
The airflow in the bypass passage portion 917 forms a flow distribution perpendicularly to the direction of the airflow. The flow speed of air is high in the vicinity of the center of the bypass passage portion 917 in the flow distribution. The flow speed of air is substantially constant, i.e., flat in the vicinity of the center of the bypass passage portion 917 in the flow distribution. Therefore, the flow speed of air does not greatly change around the center of the bypass passage portion 917 in the flow distribution.
In this structure, the thermal sensing element including the thermal resistive element 931 and the temperature sensing element 932 are preferably arranged in the vicinity of the center of the bypass passage portion 917 for detecting the flow rate of air. When the thermal sensing element is arranged in the vicinity of the bypass passage portion 917, the detection signal of the thermal sensing element does not greatly disperse, even when the position of the thermal sensing element slightly deviates from specific positions. However, when the diameter of the bypass passage portion 917 is reduced, the thermal sensing element may be originally arranged respectively in designated positions eccentrically displaced from the center of the bypass passage portion 917. In this structure, for example, when the thermal sensing element is slightly misaligned from the designated positions during assembly, dispersion in the detection signals becomes large compared with the structure in which the thermal sensing element is arranged in the center of the bypass passage portion 917. Accordingly, small misalignments of the thermal sensing element cause large dispersion in the detection property of the flow meter.